Aircraft manufacturers are under continual pressure to reduce the noise produced by aircraft in order to satisfy increasingly stringent noise certification rules. Aircraft engines are a major contributor to overall aircraft noise. Accordingly, aircraft engines in particular have been the target of manufacturers' noise reduction efforts. Aircraft engines have been made significantly quieter as a result of advanced high bypass ratio engines. These engines derive a significant fraction of their total thrust not directly from jet exhaust, but from bypass air which is propelled around the core of the engine by an engine-driven forwardly mounted fan. While this approach has significantly reduced aircraft noise when compared with pure turbojet engines and low bypass ratio engines, engine and aircraft federal regulations nevertheless continue to require further engine noise reductions.
One approach to reducing engine noise is to increase the amount of mixing between the high velocity gases exiting the engine, and the surrounding freestream air. FIG. 1 illustrates a nozzle 20 having “chevrons” that are designed to produce this effect. Chevrons generally include certain types of serrations on the nozzle lip, typically, triangular in shape having some curvature in the lengthwise cross-section, which slightly immerses them in the adjacent flow. The chevron can project either inwardly or outwardly, by an amount that is on the order of the upstream boundary layer thickness on the inner or outer surface, respectively. In general, the chevron planform shape can also be trapezoidal or rectangular. The nozzle 20 includes a core flow duct 40 through which the engine core flow is directed, and a fan flow duct 30 arranged annularly around the core flow duct 40, through which the fan air passes. The exit aperture of the fan flow duct 30 can include fan flow chevrons 35, and the exit aperture of the core flow duct 40 can include core flow chevrons 45. The chevrons typically reduce the low-frequency noise by increasing the rate at which the engine flow streams mix with the surrounding freestream air at the length scale of the nozzle diameter. While this approach has resulted in noise reduction compared with nozzles that do not include chevrons, further noise reduction is desired to meet community noise standards.